The first estimates of the severity of the H7N9 influenza virus show that about one-third of people who were hospitalized with the infection died. And flu experts warn that the strain could reappear in the next flu season.

Last fall, 162 harbor seal pups mysteriously washed up dead on the shores of New Hampshire and Massachusetts. Puzzled, scientists conducted autopsies on five of the animals, which suggested that a respiratory infection had killed …

The H1N1 flu pandemic of 2009-10 killed an estimated 284,500 people, according to Centers for Disease Control and Prevention (CDC) scientists — an estimate that is 15 times higher than the previously recorded official death toll.